This invention is related to the field of white light interferometry or vertical scanning phase-shift interferometry. White light interferometry (WLI) is a technique that uses an interferometer to profile surfaces. A set of intensity frames is acquired from a test surface sequentially by changing the optical path difference (OPD) between the test surface and a reference surface. Either the test surface or the reference surface may be moved with respect to one another to effect this change. With a spectrally broad band or white light illumination, the temporal intensity distribution recorded at each pixel in the intensity frame in WLI has a localized interferogram whose contrast is a maximum at OPD=0, decreases rapidly as OPD increases, and diminishes at OPD greater than the coherence length of illumination.
Although most commercial interferometers using WLI are capable of measuring surfaces with nanometer precision, the technique has been used to profile opaque surfaces or surfaces without a transparent thin film on them. The reason for this is that multiple reflections produced from a test surface with transparent thin films create multiple superimposed interferograms. As a result, known methods in WLI lose the ability to profile surfaces with such superimposed interferograms.